1. Field of the Invention
This invention relates to production of methacrylic acid by oxidative dehydrogenation of isobutyric acid using contact catalysts, and more particularly to oxidative dehydrogenation of isobutyric acid using catalysts containing oxides of molybdenum, vanadium, and phosphorus. The invention also relates to catalysts useful in the oxidative dehydrogenation of isobutyric acid.
2. Description of the Prior Art
The synthesis of methacrylic acid and esters thereof which are important raw materials for the production of synthetic polymers is of great and growing economic importance. The dominant industrial method of preparing these monomers at present uses acetone as the starting material and proceeds via acetone cyanohydrin which is hydrolyzed to produce the most important monomer in the methacrylate series, methyl methacrylate. Other synthetic methods which start with isobutyraldehyde or olefins such as isobutylene, ethylene, or propylene have also attracted industrial interest.
German Offenlegungsschrift No. 25 17 148 discloses a process for producing methacrylic acid from methacrolein (derived from isobutylene or isobutyraldehyde) by gas phase catalytic oxidation with oxygen. The catalyst used in this oxidation has the formula: EQU Mo.sub.12 P.sub.a X.sub.b Y.sub.c W.sub.d O.sub.e,
wherein X represents vanadium, niobium and/or tantalum and Y represents cesium, potassium and/or thallium; a and b have values from 0.1 to 10, c has values from 0.1 to 8 and d has values from 0 to 10; the value of e depends on the valence of of the remaining atoms; and the sum of a+b+c has a value of 0.3 to 20.
A number of catalysts have been suggested for oxidative dehydrogenation of isobutyric acid to methacrylic acid.
Japanese Published Unexamined patent application, Publication No. 7,504,017 discloses a catalyst containing molybdenum, phosphorus, thallium, and oxygen as useful for the gas-phase dehydrogenation of isobutyric acid.
Japanese Published Unexamined patent application, Publication No. 7,504,014 discloses the use of molybdophosphoric acids or molybdovanadophosphoric acids treated with nickel sulfate, cobalt sulfate, or an alkali metal sulfate, as catalysts for the gas-phase oxidative dehydrogenation of isobutyric acid.
The oxidative dehydrogenation of alkanecarboxylic acids and their esters to .alpha.,.beta.-unsaturated aliphatic acids and esters using calcined iron/lead phosphate as the dehydrogenation catalyst has also been disclosed (German Offenlegungsschrift No. 24 50 878). Dehydrogenation of this class of compounds with oxygen using calcined precipitated phosphate containing bismuth, iron, and optionally lead is taught in German Offenlegungsschrift No. 21 18 904.
These prior art methods, however, are not entirely satisfactory. From the modern point of view, decisive importance is ascribed not only to capital expenditures, but also to considerations of the balance of energy and raw material costs in relation to the volume-time yield and the selectivity of the processes.
Thus, processes using contact catalysts based on iron phophate produce relatively low yields of isobutyric acid. Consequently, the unconverted isobutyric acid has to be recycled back into the process. Because of the very similar physical properties of isobutyric acid and methacrylic acid, separation of the two compounds on an industrial scale is uncertain and, in any case, expensive.
German Offenlegungsschrift No. 25 50 979 discloses the use of solid catalysts consisting essentially of molybdenum and/or tungsten or their respective oxides at temperatures between 300.degree. and 500.degree. C.
German Offenlegungsschrift No. 27 22 375 discloses a hydrothermal method of preparing catalysts based on heteropolyacids of an oxide or oxyacid of molybdenum, vanadium, phosphorus, and optionally tungsten which are suitable for oxidative dehydrogenation of isobutyric acid or its methyl ester and of methacrolein.
Studies of heterpolyacid catalysts have shown, however, that relatively high yields with good selectivity can be achieved only above 330.degree. C. and in a relatively narrow temperature range (about 330.degree. to 350.degree. C.). Below this temperature range the yields fall off abruptly, while above 350.degree. C. the catalyst is evidently irreversibly damaged so that the yield and the selectivity fall off markedly. Maintaining such a narrow temperature range at a relatively high temperature under the conditions of heterogeneous catalysis is difficult to realize in industrial practice if costs are to remain resonable. Accordingly, in practice, such catalysts have a limited life under the reaction conditions specified above.
Hence a need has continued to exist for a method of producing methacrylic acid by the catalytic oxidative dehydrogenation of isobutyric acid which has a good yield, high selectivity, and which uses a practical catalyst system.